Semiconductor devices having an integrated circuit (driver IC) for driving a liquid crystal display panel, for example, are often configured such that plural types of MOSFETs (metal oxide semiconductor field effect transistors) having different breakdown voltages are mounted on a semiconductor substrate.
FIG. 4 is a sectional view showing the basic construction of such a MOSFET.
The MOSFET includes, for example, a source region 103 and a drain region 104 provided on opposite sides of a channel region 102 in a surface of a silicon substrate 101. A gate oxide film 105 of SiO2 (silicon oxide) is provided on the channel region 102 as extending between the source region 103 and the drain region 104. A gate electrode 106 of polysilicon is provided on the gate oxide film 105.
The MOSFET having such a construction has a lower breakdown voltage on the order of 5 V, because field concentration (higher electric field) occurs in the vicinity of the drain region 104. An LDD (lightly doped drain) structure is known as capable of relieving the field concentration in the vicinity of the drain region 104. Even with the LDD structure, it is only possible to increase the breakdown voltage to about 7 V. Therefore, the MOSFET shown in FIG. 4 is generally used as a low breakdown voltage MOSFET.
A DMOSFET (double diffused metal oxide semiconductor field effect, transistor) is known as having a high breakdown voltage on the order of 10 V or higher. However, the DMOSFET is substantially different in const ruction from the MOSFET shown in FIG. 4. Therefore, where the DMOSFET and the MOSFET are to be mounted on the same semiconductor substrate, a process for fabricating the DMOSFET and the MOSFET is significantly complicated.
Further, if a surge voltage is inputted into the drain region 104 of the MOSFET having the construction shown in FIG. 4, a surge current flows to be concentrated on an end portion of the gate oxide film 105 adjacent to the drain region 104, resulting in breakdown of the end portion of the gate oxide film 105 (so-called ESD (electrostatic discharge) breakdown). The DMOSFET also suffers from ESD breakdown of its gate oxide film, because the gate oxide film contacts an epitaxial layer serving as a drain region.    Patent Document 1: JP-A-HEI11 (1999)-186543